The present invention relates to a method, to an electrical heating device, and to apparatus for heating a liquid. The invention is especially useful for vaporizing a liquid condensate produced during the operation of cooling devices, but is also useful for heating or vaporizing water in many other applications, some of which are described below for purposes of example.
The operation of cooling devices, such as air conditioners, refrigerators, and other types of heat pump machines, is generally accompanied by a condensation of water from the atmosphere. If such condensed water is not immediately removed, it will freeze and thereby reduce the operating efficiency of the heating device. At the present time, such condensed water is generally removed from air conditioners by drain tubes leading to the atmosphere or to condensate collection containers. However, there are many situations wherein removal of water condensate in such a manner is not possible or practical.
Another known technique for removing water condensate is by vaporizing the water condensate, but such known techniques generally require considerable electrical power consumption and/or relatively complicated control systems.
Thermistors are small electrical devices whose resistance decreases, and therefore whose conduction of electrical current increases, rapidly and predictably with a rise in temperature, used especially in heat measurement and in voltage regulators in communication circuits. PTC (positive temperature coefficient) thermistors are thermistors whose electrical resistance increases (rather than decreases) with a rise in temperature. PTC thermistors have a capability of providing a number of important advantages over conventional filament-type heating elements, including: simplified temperature sensing and control, large heat-transfer surfaces, longer service life since they are not subject to oxidation, and more safety particularly when used in environments containing fuel or ignitable substances.
However, PTC thermistors have number of drawbacks which have heretofore limited their use in heating applications. Thus, permanent connections to PTC thermistors, such as by soldering or welding, generally have short service lives because the differences in thermal coefficients of expansion at the connections produce strains and cracks during heating/cooling cycles. In addition, thermal gradients in PTC thermistors, due to non-uniform heat transfer or exposure to air or gas flow, create a “pinch effect” which reduces heat generation efficiency and service life. Further, high local electrical field strengths due to sharp edges or ridges may trigger an electric arc that can destroy the thermistor.
The prior art includes many patents directed to positive temperature coefficient thermistors, and to various applications of such thermistors. U.S. Pat. Nos. 5,598,502 and 5,471,034 disclose PTC thermistor devices for heating liquids wherein the PTC heating elements are enclosed. However, in both cases, the disclosed devices have thermal resistance between the PTC elements and the heat exchange members that reduces their heat transfer efficiency, and thereby their power output. In addition, such devices have the additional disadvantage of not being fully immersible and/or of requiring channels for the liquid through heat transfer members, which complicates their construction and integration into liquid systems to be heated. PTC devices are also described in U.S. Pat. Nos. 4,529,868; 4,644,316; 4,677,280; 4,954,692; 5,198,640; 5,337,038; 5,377,298; 5,520,892; 5,710,459; 6,025,771; Japanese 2-191303; and Japanese 3-74803.